1. Field of the Invention
The invention is related generally to apparatus for measuring and recording environmental conditions, and in particular it concerns electrical logging apparatus for use in bore holes in the earth for sensing, digitizing and storing values of down hole pressure and temperature conditions.
2. Description of the Prior Art
It is necessary to measure and record environmental conditions in a number of important activities conducted both above and below ground. In the petroleum production industry, for example, the measurement and recording of down hole pressure and temperature conditions is of great importance in the location of petroleum bearing strata. The presence and thickness of oil and water layers below the surface of the earth can be determined by the pressure and temperature gradient existing between different depths along the bore hole. The time rate of decrease in pressure at the bottom of the well during fluid production, and the rate of recovery in pressure when production is terminated are indicative of the geological characteristics of the oil bearing strata. The value of this environmental information is dependent upon the accuracy of the sensors and the fidelity of the data storage apparatus in which the data are recorded.
The down hole pressure and temperature measuring devices known in the art have generally relied upon electro-mechanical analog systems to record the desired data. The pressure sensor most commonly used in the mechanical analog system is a high torque Bourdon tube by means of which pressure is sensed via the expansion and contraction of a movable element of the tube. A stylus attached to the movable element of the tube is used to record the information on a chart which is rotated by a mechanical cylinder and clock mechanism. For temperature measurement, a bimetallic helix is commonly used as a temperature sensor in the same way that a Bourdon tube is used for pressure measurements. The paper which contains the pressure or temperature information constitutes a permanent analog memory. Therefore, uniform chart motion and accurate stylus response are essential for accuracy. The information stored in the permanent analog memory is translated into digital form after the apparatus is retrieved from the bore hole.
Another analog measuring technique known in the art is by the measurement of current conducted through a length of electrical wire from a measuring device disposed at the proper down hole depth to a remote indicator or recorder which monitors the measurement. The accuracy of such a measurement is dependent in part on the resistance of the length of the wire which in turn is dependent upon the total length of the wire from the sensing device to the indicator. The total length of the wire may vary from well to well due to the depth of the bore hole and the distance of the indicator or recorder from the hole. Furthermore, the resistance of the electrical wire may vary due to temperature variations in the hole.
Because of the foregoing limitations associated with analog measuring and recording techniques, improved measuring and recording apparatus utilizing digital storage techniques have been developed. An example of apparatus for down hole pressure and temperature measurement in which a random access digital data memory is used to store pressure and temperature data is disclosed in U.S. Pat. No. 4,033,186. A further advance in improving the accuracy of the sensing apparatus is disclosed in U.S. Pat. No. 3,968,691, entitled "Environmental Condition Sensing Apparatus", by Hayati Balkanli, in which radiant energy sources and sensors, as well as a small coded wheel are used in place of certain analog elements, which is believed to reduce the size of the apparatus while increasing its accuracy.
Although the prior art approaches represent significant improvements in the measuring and recording of environmental condition data, a significant error may be introduced by the pressure and temperature anomalies associated with the transducers and encoding equipment commonly utilized in these systems. Experience has shown that environmental condition sensors, and in particular pressure transducers are subject to certain pressure responsive anomalies which may distort the actual value of the pressure condition being sensed, and also that they may be subject to a temperature sensitive anomaly which may also distort the value of the pressure condition being sensed. These anomalies, if not compensated, may introduce significant error into the data gathered by the system.
Therefore, it is an object of this invention to provide an environmental condition sensing and recording apparatus which will sense down hole pressure and temperature, or any two environmental conditions, digitize the information precisely and record the information in a digital data storage unit.
It is a further object of this invention to provide an environmental condition sensing apparatus utilizing sensors and data processing elements which include calibration circuitry means for removing or compensating certain environmental condition sensitive anomalies to which the sensors are responsive.